Ion exchange membranes (IEM) are used in fuel cells as solid electrolytes. A membrane is located between the cathode and anode and transports protons formed near the catalyst at the hydrogen electrode to the oxygen electrode thereby allowing a current to be drawn from the cell. These polymer electrolyte fuel cells are particularly advantageous because they operate at lower temperatures than other fuel cells. Also, these polymer electrolyte fuel cells do not contain any corrosive acids which are found in phosphoric acid fuel cells.
Ion exchange membranes are also used in chloralkali applications to separate brine mixtures to form chlorine gas and sodium hydroxide. The membrane selectively transports the sodium ions across the membrane while rejecting the chloride ions. Additionally, IEMs are useful in the areas of diffusion dialysis, electrodialysis and for pervaporation and vapor permeation separations.
In electrodialysis, electrolytes can be divided into a concentrated and a diluted stream. This is accomplished by arraying anionic and cationic exchange membranes in a filter press arrangement. Alternating compartments between the membranes are filled with either the feed stream or the product stream. An electric field is applied across this series array by inserting electrodes in the end compartments. At the positive electrode, oxygen is produced, as well as hydrogen ions. At the negative electrode, hydrogen is evolved as well as hydroxide ions.
In diffusion dialysis, a stream of contaminated acid or base can be separated from dissolved metal ions, colloidal or non-ionic species. The acid or base can than be returned to the original process. A diffusion dialysis system consists of a filter press type arrangement with anion or cation exchange membranes between compartments of that system. Alternate compartments are filled with either the waste material or water. The desired ions diffuse through the membrane. The undesired ions are rejected and removed as waste.
The IEMs must have sufficient strength to be useful in their various applications. Often this need for increased strength requires the membranes to be made thicker which decreases their ionic conductivity. For example, IEMs that are not reinforced (such as those commercially available from E. I. DuPont de Nemours, Inc., and sold under the registered trademark Nation.RTM.) are inherently weak, and must be reinforced at small thicknesses (e.g., less than 0.050 mm) with additional materials causing the final product to have increased thickness.
U.S. Pat. No. 3,692,569 to Grot relates to the use of a coating of a copolymer of fluorinated ethylene and a sulfonyl-containing fluorinated vinyl monomer on a fluorocarbon polymer that was previously non-wettable. The fluorocarbon polymer may include tetrafluoroethylene polymers. This coating provides a topical treatment to the surface so as to decrease the surface tension of the fluorocarbon polymer. U.S. Pat. No. 4,453,991 to Grot relates to a process for making a liquid composition of a perfluorinated polymer having sulfonic acid or sulfonate groups in a liquid medium that is contacted with a mixture of water and a second liquid, such as a low molecular weight alcohol. The liquid made by the process may be used as a coating, a cast film, or as a repair for perfluorinated ion exchange films and membranes.
U.S. Pat. No. 4,453,991 to Grot relates to a process for making articles coated with a liquid composition of a perfuorinated polymer having sulfonic acid or sulfonate groups in a liquid medium by contacting the polymer with a mixture of 25 to 100% by weight of water and 0 to 75% by weight of a second liquid component, such as a low molecular weight alcohol, in a closed system.
U.S. Pat. No. 4,469,744 to Grot et al. relates to a protective clothing of fabric containing a layer of highly fluorinated ion exchange polymer. Example 1 refers to a microporous polytetrafluoroethylene (PTFE) film having a thickness of 127 micrometers as described by U.S. Pat. No. 3,962,153. Solution is applied with the use of a vacuum. The film was then placed in an oven (under vacuum) at 120.degree. C. for 5 hours. The final product had a thickness of about 127 micrometers (5 mils) and required the use of a vacuum to provide for any impregnation.
U.S. Pat. No. 4,902,308 to Mallouk, et al. relates to a film of porous expanded PTFE having its surfaces, both exterior and internal, coated with a metal salt of perfluoro-cation exchange polymer. The base film of porous, expanded PTFE (ePTFE) had a thickness of between 1 mil and 6 mils (0.025-0.150 mm). The final composite product had a thickness of at least 1 mil (0.025 mm) and preferably had a thickness of between 1.7 and 3 mils (0.043-0.075 mm). The composite product was permeable to air and the air flow, as measured by the Gurley densometer ASTM D726-58, was found to be between 12 and 22 seconds.
U.S. Pat. No. 4,954,388 to Mallouk, et al. relates to an abrasion-resistant, tear resistant, multi-layer composite membrane having a film of continuous ion exchange polymer attached to a reinforcing fabric by means of an interlayer of porous expanded PTFE. A coating of a ion exchange resin was present on at least a portion of the internal and external surfaces of the fabric and porous ePTFE. The composite membrane made in accordance with the teachings of this patent resulted in thicknesses of greater than 1 mil (0.025 mm) even when the interlayer of porous ePTFE had a thickness of less than 1 mil (0.025 mm).
U.S. Pat. No. 5,082,472 to Mallouk, et al. relates to a composite membrane of microporous film in laminar contact with a continuous ion exchange resin layer wherein both layers have similar area dimensions. Surfaces of internal nodes and fibrils of ePTFE may be coated at least in part with ion exchange resin coating. The membrane of ePTFE had a thickness of about 2 mils (0.050 mm) or less and the ion exchange layer in its original state had a thickness of about 1 mil (0.025 mm). The ePTFE layer of this composite membrane imparted mechanical strength to the composite structure and the interior of the ePTFE was preferably essentially untilled so as to not block the flow of fluids.
U.S. Pat. Nos. 5,094,895 and 5,183,545 to Branca, et al. relate to a composite porous liquid-permeable article having multiple layers of porous ePTFE bonded together and having interior and exterior surfaces coated with an ion exchange polymer. This composite porous article is particularly useful as a diaphragm in electrolytic cells. The composite articles are described to be relatively thick, preferably between from 0.76 and 5 mm.
U.S. Pat. No. 4,341,615 to Bachot, et al. relates to a fluorinated resin base material treated with a copolymer of an unsaturated carboxylic acid and a non-ionic unsaturated monomer for use as a porous diaphragm in the electrolysis of alkaline metal chlorides. The fluorinated resin base material may be reinforced with fibers, such as asbestos, glass, quartz, zirconia, carbon, polypropylene, polyethylene, and fluorinated polyhalovinylidene (col. 2, lines 13-17). Only 0.1 to 6 percent of the total pore volume of the support sheet is occupied by the carboxylic copolymer.
U.S. Pat. No. 4,604,170 to Miyake et al. relates to a multi-layered diaphragm for electrolysis comprising a porous layer of a fluorine-containing polymer, having a thickness of from 0.03 to 0.4 mm with its interior and anode-side surface being hydrophilic and an ion exchange layer on its cathode surface. The ion exchange layer is thinner than the porous layer, with a thickness of at least 0.005 mm, and the total thickness of the diaphragm is from 0.035 to 0.50 mm.
U.S. Pat. No. 4,865,925 to Ludwig, et al. relates to a gas permeable electrode for electrochemical systems. The electrode includes a membrane located between, and in contact with, an anode and a cathode. The membrane, which may be made of expanded polytetrafluoroethylene, may be treated with an ion exchange membrane material with the resulting membrane maintaining its permeability to gas. Membrane thicknesses are described to be between 1 and 10 mils, (0.025-0.25 mm), with thicknesses of less than 5 mils (0.125) to be desirable. Examples show that membrane thicknesses range from 15 to 21 mils.
Japanese Patent Application No. 62-240627 relates to a coated or an impregnated membrane formed with a perfluoro type ion exchange resin and porous PTFE film to form an integral unit. The resulting composite is not fully occlusive. Furthermore, the teachings of this patent do not provide for permanent adhesion of the ion exchange resin to the inside surface of the PTFE film. The weight ratio of the ion exchange resin to PTFE is described to be in the range of 3 to 90% with a preferable weight ratio of 10 to 30%.
Japanese Application Nos. 62-280230 and 62-280231 relate to a composite structure in which a scrim or open fabric is heat laminated and encapsulated between a continuous ion exchange membrane and an ePTFE sheet thus imparting tear strength to the structure.
Additional research has also been conducted on the use of perfluorosulfonic acid polymers with membranes of expanded porous polytetrafluoroethylene such as that described in Journal Electrochem. Soc., Vol. 132, No. 2, February 1985, p. 514-515. The ion exchange material was in an ethanol based solvent without the presence of water or surfactant. Moreover, ultrasonic energy was used in the treatment of this membrane.
A paper titled "Ion Transporting Composite Membrane", by Lui & Martin, (Journal Electrochemical Society, Vol., 137, No. 2, February 1990) describes doping a microporous host material with an ion exchange polymer for the purposes of electrocatalysis.
None of the above described materials adequately addresses the current and anticipated demands for an ion exchange membrane. There remains a distinct need for a strong, ultra-thin, integral composite ion exchange membrane, having long term chemical and mechanical stability, very high ionic conductance, and having a thickness, before swelling, of less than 1 mil (0.025 mm). Because the present invention is thinner than the membranes of prior art, the ionic conductance is substantially higher than with any other ion exchange membranes.